In recent years, organic electronic devices have drawn increasing attention. The reasons are that they have flexibility, are applicable to a large area, and make it feasible to employ a low-cost, high-speed printing process in manufacturing of electronic devices. The typical organic electronic devices include organic EL elements, organic solar cell elements, organic photoelectric conversion elements and organic transistor elements. Organic EL elements intend to be applied to flat panel displays, and have been applied to mobile phone displays through TVs and the like. Organic EL elements with higher functionality have been continuously developed. Organic solar cell elements have been used as flexible, low-cost energy sources, and organic transistor elements have been applied to flexible displays and low-cost IC's. Research and development thereof have been aggressively carried out.
In developing these organic electronic devices, it is very important to develop organic semiconductor materials constituting the devices. Acene organic semiconductors such as pentacene have been investigated as an organic transistor material. Heterocyclic compounds such as heteroacene compounds, in particular the compounds containing sulfur or selenium atoms, have been also investigated. Among them, benzothieno-benzothiophenes (e.g., DPh-BTBT and alkyl BTBT), dinaphtho-thienothiophenes (DNTT) and the like have been developed as high-performance materials with stability in atmosphere and have been proposed as compounds with excellent semiconductor characteristics and stability in comparison with pentacene (Patent Literatures 1 to 3 and Non Patent Literatures 1 to 3). Regarding methods of manufacturing these useful compounds, there are many reports. However, the methods are not satisfactory due to unsatisfactory yields and the difficulty in producing a compound with an asymmetrical structure at a high yield. Further improvement in the manufacturing method is desired.
Conventionally, many methods have been tried to synthesize [1]benzothieno[3,2-b][1]benzothiophene (hereinafter abbreviated as BTBT).
Regarding a method of synthesizing BTBT, Patent Literature 4 discloses a method of synthesizing BTBT by a reaction between α,α-dichlorotoluene and sulfur. However, a compound having a dichloromethyl group generally has many problems in availability and storage stability. Patent Literature 4 and Patent Literature 5 disclose a method of synthesizing BTBT from α,α,α-trichlorotoluene, but the method causes a very low yield of 12% and thus is not practicable. Thereafter, a synthesis method represented by the following reaction formula 1 has been developed and known. However, the method has a problem of very high production cost due to the long reaction path (Patent Literature 6).

A method of producing the compound comprising the steps of reacting 2,2′-dibromodiphenylacetylene with tert-butyl lithium at extremely low temperatures and then adding sulfur thereto has been developed. However, tert-butyl lithium, used in the method, reacts with moisture in the air to take fire. The method therefore has problems in safety and industrial applicability (Patent Literature 7 and Non Patent Literature 4).
Patent Literature 8 describes a method of synthesizing BTBT comprising diazotization of 2,7-diamino BTBT, a starting material, and diazo decomposition of the diazo BTBT. However, the synthesis of 2,7-diamino BTBT (in the reaction formula 1, R═NH2) requires several reaction steps. Use of 2,7-diamino BTBT as a starting material therefore causes high cost and inefficient manufacturing methods.
BTBT may be fused with a benzene ring. As such a fused compound, for instance, dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (hereinafter abbreviated as DNTT) has been known.
It has been reported that this compound has excellent characteristics as organic semiconductor. There is therefore need to establish an industrial method of manufacturing DNTT derivatives.
Patent Literature 9 and Non Patent Literatures 3 and 5 describe a method of synthesizing DNTT through the following reaction scheme.

The synthesis method, however, has various problems: (1) in Step 1, use of dimethyl disulfide, i.e. a malodorous substance; (2) in Step 1, use of n-butyl lithium, i.e. a water prohibitive substance; (3) in Step 3, a large amount of iodine and the like are required and reaction efficiency is extremely low; and (4) by-products such as methyl iodide, i.e. a deleterious substance are generated, which cause environmental problems.
In view of the foregoing, it is extremely difficult to industrially produce BTBT derivatives (including DNTT derivatives). Further improved manufacturing methods have been therefore investigated.
Patent Literature 10 discloses a method of synthesizing BTBT wherein an aromatic aldehyde is used as a starting material, and in Patent Literature 11 discloses a method of synthesizing BTBT wherein a halogeno-aromatic aldehyde is used as a starting material. These methods allow BTBT derivatives (including DNTT derivatives) to be synthesized in one step from an aromatic aldehyde (refer to the following reaction formula 3).

Patent Literature 12 discloses a method of synthesizing BTBT from stilbene derivatives. This method allows stilbene derivatives having various substituents to be employed as a starting material, and allows a sulfur atom to be selectively introduced to the position of a leaving group X of a stilbene derivative, thereby determining the condensation position. Consequently, BTBT derivatives (including DNTT derivatives) can be relatively easily synthesized at a high yield (refer to the following reaction formula 4).

Further, Patent Literature 13 and Non Patent Literature 6 disclose that BTBT derivatives can be produced by intramolecular cyclization reaction of a precursor obtained by Stille coupling with an acid, and dealkylation of the obtained alkyl intermediate (refer to the following reaction formula 5). However, these methods cause high cost to produce the intermediate and also have the problem that producible compounds are limited

As described above, it can be easily presumed that BTBT derivatives (including DNTT derivatives) are a group of compounds having excellent characteristics. Industrial manufacturing methods thereof still are being investigated.